Photopolymerizable compositions and photopolymerizable inkjet inks using (meth)acrylic acid esters have been widely known (see PTL1).
However, many of monomers used in conventional photopolymerizable inkjet inks are toxic. Especially, most of (meth)acrylic acid esters, which are readily available with low cost, have high toxicity in terms of skin sensitivity, which causes allergy reactions with skin upon contact with them. Conventional art has not yet provided any solution to this problem.
Based on the studies conducted so far, the present inventors have found a few (meth)acrylic acid ester and (meth)acryl amide, which have no problem in skin sensitization. As for one of examples thereof, proposed is an inkjet ink containing methacrylate as a main component, as methacrylate is lower in toxicity in terms of skin sensitization than acrylate, as disclosed in Japanese Patent Application No. 2010-278177, and PTL 2.
However, in order to further improve performances, it has been desired to improve a strength of a cured coating film to enhance durability thereof against abrasions or the like, and to reduce a viscosity so that it can be ejected by inkjet without any problem even when an ink increases its viscosity as a result of formulated various materials, such as a pigment, and additives, therein. To reduce viscosity of an ink, water can be blended therein, and a photopolymerizable aqueous inkjet ink, to which water is blended, has been known. As described later, however, such ink is not penetrated and dried on a non-absorbent base. Use of such ink requires a step for evaporating water in order to increase speed or attain efficiency of a process. Moreover, a heat source is desired to be provided. Therefore, it is not preferable in view of energy saving. In the case where the photopolymerizable composition is used as a coating material used for brush coating, an organic solvent can be used therein. However, the organic solvent is evaporated and released in the atmosphere. In view of a possible influence to the environment, use of the organic solvent is preferably as little as possible. In the case where the photopolymerizable composition is used as an inkjet ink, if an organic solvent blended therein is highly volatile, the ink in a nozzle, through which the ink is jetted, increases its viscosity due to vaporization of the organic solvent, which may cause a problem in jetting. Therefore, such ink is not preferable.
In view of these points mentioned above, the present inventors have invented technologies disclosed in Japanese Patent Application Nos. 2012-46301 and 2012-113970. However, further improvement in a coating strength thereof has been desired to resist under more severe conditions, as it is used to protect surfaces of housing or molded products. Therefore, it has been desired to attain a coating film having higher durability than that disclosed in Japanese Patent Application No. 2012-46301. As for the durability of the coating film formed to protect surfaces, it is also required, other than sufficient solidness, that an appearance does not change even when it is exposed to direct sun light.
Moreover, if a solid coating film is produced using the photopolymerizable composition, not by printing a certain image pattern, not only an inkjet system, but also a method, such as spray coating, and brush coating, can be employed. It is however difficult to provide a coating film, which has no problem of skin sensitization, and has high strength and excellent light fastness, regardless of a coating system.